Résumé : Numerous experimental studies on the β-adrenergic receptor suggested that the adenylate cyclase activity is correlated to the number of agonist-receptor complexes. In S49 lymphoma cells, it has been previously shown that, for a given number of occupied receptors, the level of adenylate cyclase activity depends on the total number of receptors. Here we propose and demonstrate that such a behaviour can be accounted for by the cyclic model of Cassel and Selinger provided the diffusion of the receptor in the plane of the membrane is explicitly introduced in the model. The key properties of this expanded version of the model are related to the relative kinetics of receptor diffusion and of agonist dissociation from the receptor.